The present invention relates to a device for detecting the viscosity of specific gravity of liquid using a vibrator comprising piezoelectric ceramics as a drive source.
The inventors have proposed a vibrator made of piezoelectric ceramics that converts mechanical energy to electrical energy as disclosed in Japanese Patent Application No. 61-299598. The related art vibrator vibrates about an axis thereof. When the vibrator is a parallelepiped, for example, adjacent corners of the vibrator vibrate in opposite directions and each of the corners repeatedly vibrate in alternate directions parallel to the thickness of the vibrator. They have considered the applicability of this vibrator to a device for detecting the viscosity or specific gravity of liquid and have conducted experiments in which the vibrator was connected to a detection terminal for the purpose of vibrating the detection terminal both in air and in liquid and converting the difference in vibration of the detection terminal when in the air and in the liquid into an equivalent electric signal through its own conversion function. In other words, the vibrator has been used both as a drive source for the detection terminal and as a sensor for sensing a variation in the vibration of the detection terminal and converting the variation into an equivalent electric signal.
The vibrator serving as the aforementioned drive source is vibrated about its axis at a resonance frequency of a vibration system including itself and the detection terminal. When detecting the viscosity of liquid, a variation in voltage level at the time of resonance within the liquid is detected as electric signal indicative of the viscosity. When detecting the specific gravity of liquid, the variation in resonance frequency of the detection terminal within the liquid is detected as an equivalent electric signal indicative of the specific gravity.
With the application of the vibrator serving both as a drive source and as a sensor to a device for detecting the viscosity or specific gravity of liquid, it is possible to obtain measurement with some degree precision. However, the device is subject to unstable operation resulting from disturbance of the resonance point and detection errors resulting from environmental conditions, such as temperature, which produce a fluctuation in the output of a bridge circuit through which the aforementioned vibrator and a condenser are connected to each other.